This invention relates, in general, to a phase and amplitude detector and a method of determining errors, and is particularly, but not exclusively, applicable to the measurement of phase and amplitude errors for compensation purposes in the linearisation of power amplifiers.
First and second generation cellular systems have historically used forms of signal modulation which are either constant envelope (e.g. Gaussian Minimum Shift Keying (GMSK) in the global system for mobile communication (GSM) or which result in relatively low levels of amplitude modulation. The linearity of the high power amplifiers used for such systems has therefore not been an important technical issue. Indeed, for constant envelope systems, it is standard practice to operate amplifiers either close to or actually in compression in order to maximise power efficiency. That is to say, the amplifiers are intentionally employed in a non-linear mode.
Third generation cellular systems, however, typically use linear spread-spectrum modulation schemes with a large amount of amplitude modulation on the signal envelope. When passed through a high power amplifier, the output is typically distorted in amplitude and phase by the inherent non-linearity of the amplifier. The amplitude and phase distortion effects are commonly referred to as AMxe2x80x94AM conversion and AM-PM conversion, respectively. Both distortion effects are principally a function of the amplitude envelope of the input signal and are insensitive to the input phase envelope.
In Code Division Multiple Access (CDMA) modulation schemes, quadrature amplitude modulation (QAM) and systems employing similar linear transmission mechanisms, a plurality of signals are simultaneously amplified and transmitted which cause the generation of a large amplitude component in the signal envelope. Unfortunately, when a large amplitude component is applied to a linear amplifier, its non-linear characteristics will tend to produce intermodulation products that reduce signal quality and can cause spectral spillage outside a particular licensed spectrum. Intermodulation products must, therefore, be controlled, but such control, as will be appreciated, should not be at the expense of reducing wanted signal strength.
Intermodulation products and associated distortion can be reduced by negative feedback of the distortion components, pre-distortion of the signal to be amplified to cancel the amplifier generated distortion, or by separating the distortion components from the amplifier output and feeding forward the distortion components to cancel the distortion of the amplifier output signal.
In a power amplifier, where linearisation is performed by correction as a function of signal envelope (either via feedback or via pre-distortion), there is a need for an accurate amplitude and phase comparator that can operate over the full dynamic range of the input signal. In addition, it is desirable for the detector to have a high processing speed to cope with wideband spread spectrum signals. In other words, whilst maintaining low cost and high efficiency design, power amplifiers require ancillary error detection circuitry that can identify and allow correction for non-linearity. Indeed, such correction circuitry is critically dependent upon an ability to measure accurately the phase and amplitude of both the input and output signals to the power amplifier, which signals generally (and, in the exemplary case of CDMA-based systems, inherently) have signal envelopes with associated large dynamic ranges (typically xcx9c20 decibels). In fact, with this ancillary error detection circuitry, there is a requirement to measure small error components (typically of the order of a few tenths of a decibel) in amplitude and phase with respect to relatively large wanted signal excursions/envelopes.
Typical amplifier architectures incorporate a slow feedback loop to track out unit-to-unit variations, thermal drift and long-term component drift. The slow feedback loop eases amplifier set-up and allows a fast feedback or a pre-distortion mechanism to operate only on the amplifier induced, envelope-dependant distortion components. However, conventional phase and amplitude detectors of sufficient performance (associated with linearisation and specifically phase and amplitude error correction in a fast loop) have proven to be extremely difficult to set-up and to replicate on a commercial basis. In any event, it is desirable that a common detector mechanism is used to close both the fast error loop and the (somewhat auxiliary) slow feedback loop to ensure that both loops converge on a single phase/amplitude state.
In accordance with a first aspect of the present invention there is provided a detector operable to provide at least one error signal associated with at least one of a phase error term and an gain error term between a reference signal R and a feedback signal F, the detector characterised by: a vector generator responsive to the reference signal R, the vector generator producing a frame of reference vectors R1-Rn generated by a combination of the reference signal R with first A and second P offset vectors that provide an amplitude and phase displacement of the reference signal R; a signal combiner arranged to generate difference vectors E1-En by combining the frame of reference vectors R1-Rn and the feedback signal F, the difference vectors E1-En. expressing the phase (p) and the gain (a) error terms relative to the reference signal R and the first A and second P offset vectors; and an error signal detector responsive to the difference vectors E1-En and arranged to provide a measure of the phase (p) and the gain (a) error terms required to support subsequent generation of the at least one error signal.
In a preferred embodiment, a combinatory circuitry coupled to the error signal detector is arranged to receive output signals from the error signal detector, the combinatory circuitry configured to isolate the phase error term and the gain error term in terms of the first A and second P offset vectors and the reference carrier vector R.
Preferably, the combinatory circuitry generates the at least one error signal through isolation of the phase error term from the gain error term, the at least one error term satisfying the general form:
X=P1xe2x88x92P2xe2x88x92P3+P4=xe2x88x928PpR; 
Y=P1+P2xe2x88x92P3xe2x88x92P4=xe2x88x928AaR
where a is the gain error term, p is the phase error term and Pn are output amplitudes from the signal error detector for corresponding difference vectors E1-En.
In another aspect of the present invention there is provided a phase and amplitude comparator operable to provide signals relating to the difference in phase and amplitude between a reference signal R and a feedback signal F, wherein the comparator comprises vector generating means to produce four reference vectors R1-Rn which are related to the input reference vector signal R by the addition of further vectors xc2x1A and xc2x1P which are, respectively, in phase and in quadrature with R such that:
R1=R+A+P; 
R2=R+Axe2x88x92P; 
R3=Rxe2x88x92Axe2x88x92P; 
R4=Rxe2x88x92A+P; 
wherein the four reference vectors R1-Rn are added to four samples of the feedback signal F to produce four corresponding error vectors E1-E4 whereby the vectors E1-E4 can be used to generate phase (X) and amplitude (Y) comparative signals.
In another aspect of the present invention there is provided an amplifier circuit comprising: an input coupled to receive, in use, a reference signal R; phase and gain modulators coupled to the input; an amplifier coupled to the phase and gain modulators; a first directional coupler coupled to the input and arranged to sample the reference signal R; a second directional coupler coupled to the amplifier and arranged to sample an amplified version of the reference signal R, thereby to provide a feedback signal F; and a detector according to the first aspect, the detector coupled to the first directional coupler and the second directional coupler to receive, in use, the reference signal R and the feedback signal F; wherein the phase and gain modulators are arranged to receive phase and gain corrections signals derived from the at least one error signal (Y, X) generated by the detector.
In a preferred embodiment the amplifier circuit further comprises an adaptive pre-distorter coupled to receive the at least one error signal from the detector, the adaptive pre-distorter further coupled to the phase and gain modulators, the adaptive pre-distorter arranged to determine the gain and phase error correction signals with respect to a set of look-up values, thereby to linearise performance of the amplifier.
Preferably, a slow feedback loop containing a phase/amplitude equalizer having a second amplitude modulator and a second phase modulator coupled to the amplifier, the phase/amplitude equalizer further containing baseband processing elements coupled to the detector and arranged to receive, in use, the at least one error signal as a control signal for the baseband processing elements, whereby the phase/amplitude equalizer is arranged to track out circuit variations arising from at least one of unit-to-unit variations, thermal drift and long-term component drift through amplitude and phase control of, respectively, the second amplitude modulator and the second phase modulator.
The phase amplitude equalizer may further include: a quadrature to amplitude/phase (R, xcex8) domain converter coupled to receive the at least one error signal and arranged to provide distinct phase angle xcex8 and amplitude R components; a phase integrator coupled to the quadrature to amplitude/phase (R, xcex8) domain converter and arranged to receive, in use, the phase angle xcex8 component, thereby to provide a first time-integrated signal having a wrap-around phase correction function; an amplitude integrator coupled to the quadrature to amplitude/phase (R, xcex8) domain converter and arranged to receive, in use, the phase angle xcex8 component, thereby to provide a second time-integrated signal; and an amplitude/phase (R, xcex8) domain to quadrature converter coupled to the phase integrator and the amplitude integrator and arranged, in use, to combine the first time-integrated signal and the second time-integrated signal to exercise control of the slow feedback loop.
The amplifier circuit may have at least one delay line operable to compensate for any delay skew induced by processing delay in a correction path between the reference signal and correction signals.
The detector, the phase and amplitude comparator or the amplifier circuit may be incorporated within a base station or a subscriber unit of a cellular communication system or other signaling scheme requiring linear performance.
In a further aspect of the present invention there is provided a method of detecting at least one of a phase error term and an amplitude error term between a reference signal R and a feedback signal F and generating a corresponding error signal in response to the least one of the phase error term and the amplitude error term, the method characterised by: producing a frame of reference vectors R1-Rn generated by a combination of the reference signal R with first A and second P offset vectors that provide an amplitude and phase displacement of the reference signal R; generating difference vectors E1-En by combining the frame of reference vectors R1-Rn and the feedback signal F, the difference vectors E1-En expressing the phase and the amplitude error terms relative to the reference signal R and the first A and second P offset vectors; and providing a measure of the phase and the amplitude error terms in response to the difference vectors E1-En, the phase and the amplitude error terms required to support subsequent generation of the at least one error signal.
In a particular embodiment, the method further comprises: generating the at least one error signa through isolation of the phase error term from the amplitude error term, the at least one error term satisfying the general form:
X=P1xe2x88x92P2xe2x88x92P3+P4=xe2x88x928PpR; 
Y=P1+P2xe2x88x92P3xe2x88x92P4=xe2x88x928AaR
where a is the amplitude error term, p is the phase error term and Pn are output amplitudes from the signal error detector for corresponding difference vectors E1-En.
The detector of the present invention and its corresponding method of operation may be employed within, for example, a cellular base station or the like to improve linearity.
The present invention therefore provides an improved phase and amplitude comparator particularly, but not exclusively, useful in an amplifier linearisation process. In overview, the preferred embodiments of the present invention operate to isolate small error terms from large signal terms and then to cause corrective operation on the small error terms only. In accordance with the preferred embodiments of the present invention, an improved linear power amplifier is beneficially provided in which linearisation is performed by correction to the signal envelope. Indeed, in contrast with prior art systems, the present invention advantageously overcomes two effects exhibited by conventional phase and amplitude comparator techniques, namely an ability to resolve accurately small differences between relatively large signals with high dynamic range and, second, an ability to reduce dynamic range requirements of detectors employed to ease their associated tracking requirements.
While the detector of the preferred embodiment is optimized to resolve small signal error/offsets in large dynamic ranges, the detector can, beneficially, still provide useful output even when offsets are large. Consequently, the present invention can be used in a complementary sense within a slow feedback loop.
The detector of the preferred embodiment is able to operate sufficiently fast to cope with wideband spread spectrum signals and, beneficially, has a generally simplified and robust circuit design.